The present invention relates to systems for radio communication with mobiles. It lies within the receivers used in the fixed or mobile stations of such systems and operating coherent demodulation of spread spectrum signals.
Coherent demodulation requires various parameters representing the propagation channel between the transmitter and the receiver. Some of these parameters vary relatively slowly and can be estimated by statistical probing methods. Such is the case for example for the delays assigned to the multiple propagation paths in the conventional rake receiver. The delays specific to the various paths can be updated at fairly low frequency, for example of the order of a second. On the other hand, other parameters have abrupt variations, on the scale of the duration of an information symbol, due to the fading phenomenon. Such is the case in particular for the instantaneous amplitudes of reception of the symbols along the propagation paths taken into consideration, which are required for coherent demodulation. These instantaneous amplitudes are complex amplitudes, manifesting the attenuation and the phase shift undergone at each instant along the paths.
In general, these complex amplitudes are estimated from symbols known a priori, or pilot symbols, interspersed among the transmitted information symbols so as to allow coherent demodulation. This mode of estimation does not permit allowance for the fast channel variations between the patterns of pilot symbols.
An object of the present invention is to improve the consideration of fast fading in coherent demodulation schemes.